What is your current gpu? How does it handle gaming on that monitor now? Can you get by for a few months at lower resolutions if need be?
At this point in their lifecycles, I wouldn't go less than a 1070 or Vega56 for 4k gaming, even then a 1070ti/1080/Vega64 is preferable. 1060/RX580 is better for 1440p or lower, while they can produce playable framerates on low and maybe even med settings for 4k, if you keep up with newer releases, you may be disappointed in your gpu sooner than you'd hope for a new purchase.
Nvidia is supposed to have a press event on August 20th and it is assumed the 11xx series release date will be given, but who knows. So if your current card is getting you by, maybe wait a week or so and see what they say. You...